In an automatic power transmission for an automotive vehicle, the gear ratios in the forward drive ranges available of the power transmission are shifted between each other by selectively actuating fluid operated frictional units including, for example, a high-and-reverse clutch, a forward drive clutch, a brake band and a low-and-reverse brake which are all operated by a hydraulic fluid pressure delivered from a hydraulic control system. In a known power transmission using these four frictional units, the forward drive clutch is maintained in a coupled condition throughout the automatic and manual forward drive ranges of the transmission. When only the forward drive clutch is in play in the transmission mechanism having the four frictional units incorporated therein, the first or "low" gear ratio in the automatic drive range is established in the transmission mechanism. If the brake band is actuated to apply with the forward drive clutch held in the coupled condition, the second or "intermediate" gear ratio in the automatic or manual forward drive range is produced in the transmission mechanism. If, furthermore, the brake band is released and, in turn, the high-and-reverse clutch is actuated to couple with the forward drive clutch maintained in the coupled condition, the third or "high" gear ratio in the automatic forward drive range is achieved in the transmission mechanism. If, on the other hand, the low-and-reverse brake is actuated to apply in addition to the forward drive clutch kept coupled, then the first or "low" gear ratio in the manual forward drive range is produced in the transmission mechanism. When the first gear ratio in the manual forward drive range is established in the transmission mechanism, engine braking can be affected on the engine so that the vehicle is braked upon by the engine. The low-and-reverse brake is further operable in combination with the high-and-reverse clutch for achieving the reverse drive gear ratio in the transmission mechanism.
The forward drive clutch to be held in the coupled condition throughout the forward drive ranges of the transmission system is actuated by a fluid pressure which is maintained in a forward drive clutch actuating circuit throughout such ranges. To effect shifting between the first, second and third forward drive gear ratios, the hydraulic transmission control system for use with the transmission mechanism of the above described general nature has incorporated therein at least two shift valves consisting of a first-second gear shift valve contributing to the shifting between the first and second forward drive gear ratios and a second-third gear shift valve contributing to the shifting between the second and third forward drive gear ratios. In the hydraulic control system under consideration herein, these first-second and second-third gear shift valves are arranged in parallel with each other in respect of the above mentioned forward drive clutch actuating fluid circuit so that the fluid pressure to actuate the forward drive clutch is constantly developed in one fluid inlet port of each of the shift valves when the forward drive clutch is in the coupled condition. When, therefore, both of the first-second and second-third gear shift valves are kept closed concurrently, the fluid pressure developed in the forward drive clutch actuating fluid circuit is not allowed to pass through the shift valves and is directed solely to the fluid chamber of the forward drive clutch for producing the first gear ratio in the automatic forward drive range with all of the remaining frictional units held in the inoperative conditions or in the manual forward drive range with the low-and-reverse brake held in the applied condition. When the first-second gear shift valve is rendered from the closed or first gear ratio condition to the open or second gear ratio condition, the fluid pressure which has been extended thereto from the forward drive clutch actuating fluid circuit is passed through the shift valve and is directed to the brake-apply fluid chamber of a band servo unit for the brake band. The brake band being actuated to apply with the forward drive clutch maintained in the coupled condition, an upshift is made from the first gear ratio to the second gear ratio in the automatic or manual forward drive range. When the second gear ratio is thus established in the transmission mechanism, the second-third gear shift valve is maintained in the closed or second gear ratio condition so that the fluid pressure extended thereto from the forward drive clutch actuating fluid circuit is not allowed to pass through the shift valve. When the second-third gear shift valve is rendered into the open or third gear ratio condition, the fluid pressure in the forward drive clutch actuating fluid circuit is passed through the shift valve and is directed on the one hand to the brake-release fluid chamber of the band servo unit for the brake band and on the other hand to the fluid chamber of the high-and-reverse clutch. The fluid pressure developed in the brake-release fluid chamber of the band servo unit causes the brake band to release and the fluid pressure developed in the fluid chamber of the high-and-reverse clutch causes the clutch to couple. The brake band being released and the high-and-reverse clutch being brought into a coupled condition with the forward drive clutch maintained coupled, an upshift is made from the second gear ratio in the automatic or manual forward drive range to the third gear ratio in the automatic forward drive range. The fluid pressure to actuate the low-and-reverse clutch is supplied to the fluid chamber of the clutch by way of a fluid circuit which is arranged independently of the fluid circuit for the forward drive clutch.
Thus, both of the first-second and second-third gear shift valves are held in the respective lower gear ratio conditions thereof when the first gear ratio in the automatic or manual forward drive range is in play in the transmission mechanism. When the second gear ratio in the automatic or manual forward drive range is established in the transmission mechanism, the first-second gear shift valve is held in the higher gear ratio condition thereof and the second-third gear shift valve is held in the lower gear ratio condition thereof. When, furthermore, the third gear ratio in the automatic forward drive range is established in the transmission mechanism, both of the first-second and second-third gear shift valves are held in their respective higher gear ratio conditions. Insofar as the hydraulic control system including these shift valves is normally operative, therefore, it can not happen that the second-third gear shift valve assumes the higher gear ratio condition while the first-second gear shift valve is in the lower gear ratio condition thereof.
In the event, however, there is caused a failure in the second-third gear shift valve or in any of the fluid circuit leading to or from the particular shift valve, it may happen that the shift valve is rendered into the higher gear ratio condition thereof even when the first-second gear shift valve is held in the lower gear ratio condition thereof. If such an accident is brought about under a condition in which the first gear ratio in the manual forward drive range is in play in the transmission mechanism with the forward drive clutch held in the coupled condition and the low-and-reverse brake held in the applied condition, not only these two frictional unit but the high-and-reverse clutch is brought into the operative condition thereof with the result that a total of three frictional units are made operative concurrently. This causes the transmission mechanism to be interlocked in its entirety and may cause the power train including the transmission mechanism to be damaged or disabled from transmitting driving torque therethrough. The present invention contemplates provision of a solution to such a problem.